A light metal member to be cast-wrapped by a light metal cast article, which has a rough uneven outer surface formed by shot-blast blowing hard coarse pyramidal or sharp grains against the outer surface, has been known (Japanese Laid-open Patent Publication Hei 10-94867).
In the above-mentioned light metal member to be cast-wrapped, an outer surface of the hard coarse grain is required to have a sharp edge, in order to make the outer surface of the light metal member rough.
When the outer surface of the light metal member to be cast-wrapped is made rough using the hard grains, bottoms of the rough surface are formed in sharp ravines by sharp edges of the hard coarse grains, but tops of the rough surface are not necessarily formed in a sharp peaks. Further, it is required that mean grain size of the hard grains is 70 μm and distribution of the grain sizes is a nearly a predetermined normal distribution. If velocity of an air jet for blowing the hard coarse grains and ratio of amount of the air jet and amount of the hard coarse grain are not appropriate, a desired rough surface can not be obtained.
Since the above-mentioned hard coarse grain is a high-class corundum particle which is a fragile hard material with broken sharp edge, it is inevitable that the hard coarse grain becomes fine by the shot-blast. Therefore, in order to use the hard coarse grains after the shot-blast repeatedly, it is necessary that the hard coarse grains made fine by the shot-blast are separated and removed continuously to maintain a predetermined distribution of the grain size. This administration of the grain size is complicated.
Projections of a rough surface formed on the light metal member to be cast-wrapped may be melted by large heat capacity of the cast-wrapping light metal and metallurgically combined with the cast-wrapping light metal. However, the metallurgically combined portion is a part of the surface of the light metal member to be cast-wrapped and the projection of the rough surface is tapered, so that mechanical combining force between the light metal member to be cast-wrapped and the cast-wrapping light metal is low. Therefore, when a force for mutually separating the light metal member to be cast-wrapped and the cast-wrapping light metal acts owing to difference of thermal expansion of them, a crack is apt to be produced at a boundary portion between them to remarkably lower heat transfer between them.